The present invention relates to a transmission circuit having a plurality of amplifiers having different transmission frequency bands and, more particularly, to a transmission circuit and unnecessary radiant wave suppression method which can suppress unnecessary radiant waves produced by amplifiers during operation.
Currently, in the field of mobile communications, with the widespread use of terminals, the traffic has considerably increased, and a plurality of systems that greatly differ in their frequency bands exist. In order to improve the convenience to users, demands have arisen for a terminal that can singly receive services from these systems. Under the circumstances, mounting a common circuit having some of a plurality of usable frequencies is preferable to mounting a plurality of circuits that greatly differ in their usable frequencies in terms of cost and mount area. This is obvious because a reduction in terminal size is also a demand on the market.
To meet the above demand on the market, a mobile communication terminal has been proposed, on which a transmission circuit serving as a power amplifier circuit having a plurality of transmission frequency bands is mounted. Such a transmission circuit has, in one semiconductor die, a plurality of circuits optimized in the respective transmission frequency bands. For this reason, when a circuit in a certain transmission frequency band is transmitting a signal, another circuit in a different transmission frequency band may be affected by the circuit in use.
When a given transmission frequency band is nearly twice another transmission frequency band, in particular, the second harmonic produced by the amplifier in the lower frequency band coincides with a desired frequency in the amplifier in the higher frequency band. It is therefore essentially impossible to suppress this second harmonic in the amplifier in the higher frequency band. For this reason, when the amplifier in the lower frequency band is operating and the amplifier in the higher frequency band is not operating, unnecessary radiant waves are emitted, in a slight amount but at a level that cannot be neglected, from the output circuit of the amplifier in the higher frequency band which should not be operating, from the viewpoint of the overall mobile communication terminal.
To suppress such unnecessary radiant waves, a dual band transmission circuit like the one shown in FIG. 2 has been proposed. Referring to FIG. 2, reference numeral 11 denotes an amplifier for GSM (Global System for Mobile Communication )900 (890 to 915 MHz); 12, an amplifier for DCS(Digital Cellular System)1800 (1710 to 1785 MHz); 13, a low-pass filter (LPF) for GSM900; 14, an LPF for DCS1800 ; 15, a diplexer; and 16, an antenna. In the circuit shown in FIG. 2, the LPFs 13 and 14 are respectively connected to the output stages of the amplifiers 11 and 12 to suppress unnecessary radiant waves such as harmonic components.
In a conventional dual band transmission circuit, however, since the LPFs are directly connected to the output terminals of the respective amplifiers, in-band ripples are produced and attenuation of signals occur in a transmission frequency band. In addition, the load characteristics of amplifiers do not stabilize. Although FIG. 2 shows the dual band transmission circuit having amplifiers in two transmission frequency bands, similar problems arise in a transmission circuit having amplifiers in three or more transmission frequency bands as well.